Optical articles and sol-gel for their manufacture

ABSTRACT

An optical article, having an almost complete isotropy and dimensions equal to or lower than 500 μm, is prepared by a sol-gel procedure comprising a preliminary step wherein the mould is filled cold with a sol containing the desired oxide precursors, sol gelation, gel drying, removal of the gel from the mould and the subsequent miniaturization of the dried gel. The optical article can be used for optic telecommunications.

The present invention refers to optical components consisting of siliconoxide, as such or suitably combined, in final or nearly finaldimensions, having an almost totally isotropy and sizes equal to orlower than 500 μm; furthermore the invention relates to the means usedto carry out the preparation of such optical articles, as well as to theprocess for the preparation of both of them.

It is known that the optical materials, particularly the transparentoptical materials, are typical materials of known machining difficultyand, sometimes, troublesome preparation because their fragility andhardness which, for example, are conditioning the hot molding by causingoptical components and devices not being generally acceptable forreasons of product quality.

Usually, the methods for the production of uncommon morphology opticalelements comprise reducing suitable preforms through extremely accuracylens milling operations: These are methods requiring a very precisehandling even through the employment of suitable equipment, thissometimes being the reason of difficult repeatibility and poor processflexibility.

One solution has been achieved by producing aspheric lenses through hightemperature and pressure preparations, directly from suitable preformsof the desired optical material: however, the method has applicabilitylimits, needs sophisticated equipment and can be carried out onlythrough considerable investment.

One method for reducing costs consists in the use of organic opticalmaterial, i.e. plastic material, which can be melted and molded usingvery economical processes. However, the employment of plastic materialis sometimes the cause of dimensional defects of final optical product,because of the insufficiently controllable material shrinkage during themolding treatment.

The Applicant is able to overcome all drawbacks being in the processesfor the preparation of optical materials according to the known art, bymeans of the object defined in the European patents no. 586.013 and no.705.797 it may freely use: according to such patents, it is possible toprepare perfectly and completely isotropic optical components in theirfinal or nearly final dimensions by applying thermochemicaldensification treatment to an amorphous monolithic aerogel of silica,and/or other oxides, by the use of high precision tools and the formerpreparation of such aerogel via a so called sol-gel process during whichthe intermediate products, till the gel formation, undergo an ultrawavetreatment.

The final optical product, obtainable through tolerable investments andabsolutely repeatable methods, is characterized by very high precisionand dimensional steadfastness: however, it is such and the relevantpreparation method is advantageous when the optical article dimensionsare beyond a threshold, under which machining difficulties may arise inthe mould formation formation, particularly in the preparation of mouldsrepresenting microstructures which cannot be processed by means of thecommon machining tools such as, for example, the microlens matrix orother periodic microstructures needing a dimensional resolution of eachelement under the entrusting threshold of the existing machinery. Suchdifficulties increase in the case of preparation of optical articles inthe peculiar microoptical field, wherein positioning and lining upproblems have to solve of the magnitude order of micron and lower, andthat, furthermore, has applications other than the ones pertaining tothe traditional optic such as telecommunications, pick-up laser,semiconductor laser, optical memory.

The Applicant has now found it is possible to overcome all abovedrawbacks and to consequently prepare optical articles, till to thelowest possible dimensions, according to an economically advantageousand repeatable method, which substantially consists in a preliminararrangement of the desired dimension moulds and the subsequent usethereof in the preparation of the very optical article.

Therefore the present invention relates to a process for the preparationof moulds suitable to manufacture optical articles, as well as for thepreparation of such optical articles, these consisting silicon oxide, assuch or whit suitable additives, and characterized by an almost totalisotropy and dimensions equal to or lower than 500 μm, which processcomprises one or more of the following operations that, all together,can be continuously carried out in a fall and/or stopped at the desiredor suitable step:

-   a. preparation of an original high precision mould;-   b. possible reproduction, in a siliconic rubber or other suitable    compound, of one or more imprints, having the same sizes and a    reversed symmetry with respect to the moulds obtainable through the    preceding steps;-   c. preparation, by the employment of one or more of the products    obtained in the preceding steps, of the optical article having    reduced dimensions and reversed symmetry with respect to the    starting mould/imprint; according to a sol-gel procedure;-   d. possible preparation, inside the so obtained optical article, of    a further article again having reduced sizes and reversed symmetry,    or of imprints according to the preceding item b);-   e. and so on, possibly, through the preparation of optical articles    by sol-gel procedures and/or imprints according to b), till the    desired dimensions or, anyhow, till the lowest dimensions on the    ground of the physical limit of the process;-   f. possible separation, in relation with any step, of the imprint    and/or the article obtained in the very step.

Some of the above operative steps may be furtherly detailed:

-   a) a mould is prepared, hereinafter defined as original mould, made    by aluminum alloy or in other material with a suitable chemical    stability and able to undergo a precision machining processing;-   b) one or more imprint are prepared in siliconic rubber or other    suitable compound, having the same sizes and a reversed symmetry    (mirror image) with respect to the original mould;-   c) by the use of the so obtained imprint as a new mould (resulting    symmetrically reversed with respect to the original mould) a molded    manufactured article is prepared trough a sol-gel process in silica    glass which has lower dimensions according to the linear shrinkage    coefficient and the same symmetry as the original mould. The so    obtained silica glass can be find application either as optical    component or as a mould for a following molding operation;-   d) the so obtained manufactured article can be used as a reversed    imprint in view of a further operation to sol-gel produce an optical    article having further reduced dimensions and reversed symmetry    (mirror image) with respect to the original mould.

The use of the reversed imprint made by a siliconic rubber (or othermaterial) is a possible one and may be repeated during the processroute, if necessary or opportune according to the man skilled in theart.

The preparation of the original mould according to item a) can be thefirst operation of the fall process according to the present invention,or it may be carried out independently, and the obtained mould may besuitably stored, to be subsequently employed.

Such a preparation occurs according to techniques well known to thepeople skilled in the art, and the choice thereof substantially dependson the sizes of the mould itself. The technique grounds, starting fromthe conventional ones, are:

-   -   use of numerically controlled machines;    -   use of special machine tools provided with a “turning diamond”;    -   use of cutting machine to treat conventional optical networks;    -   microscopic geometry based technologies providing        photolitographic techniques combined with microerosion        techniques;    -   ablation through high power laser radiations.

The materials employed to prepare the original moulds commonly are metalalloys, preferably nickel/phosphorous based alloys on aluminum carriers,or aluminum alloys commercially known as “anticorodal” such as, forinstance, the ones defined as follows by UNI rules: 9006/1, 9006/2,9006/4, 9006/5, 9006/6. Of course it is possible the use of any othercarrier with the necessary workability and steadiness properties. Alsothe replication of these moulds into siliconic rubber (or othermaterial) imprints, without any risk occurring onto the original mould,is carried out according to procedures well known to the people skilledin the art. The main purpose of this preparation step is the productionof an opportune, even if not necessary, mould number in view of anysubsequent operation, as well as the maintenance of the original mould.

Inside the moulds obtained thereby, in the original one as well as inthe subsequent imprints, other manufactured articles employable asmoulds or optical articles are then produced, having reduced dimensionsand, time by time, reversed symmetry, through sol-gel procedures: thesol-gel processes are carried out according to the well knowntechnologies, on the ground of just common principles and methods, aboutwhich reference can be made to the field literature, patents too such asU.S. Pat. No. 4,317,668, No. 4,426,216, No. 4,432,956 e No. 4,806,328.

When the production, at any step of the above fall process, aims toobtain an optical article, the dried gel is removed from the mould to besubmitted to a suitable isotropic miniaturization.

The composition of the moulds and/or optical articles prepared after thestep b) of the inventive process, comprises a silicon oxide, as such oradded by oxides of elements modifying the properties thereof, inparticular way le optical properties.

An example of the modification of the optical properties of the silicaglasses consists in the refraction index increase, that is obtained by asuitable chemical formulation of the sol, which precursors of theappropriate oxide are added to, particularly titanium and/or germaniumoxides. By following the same trick, other optical properties aremodified such as the optical dispersion, through the addition ofprecursors of oxides of elements belonging to the lanthanium group. Theaddition of appropriate active oxide precursors to sol can promote alsothe modification of properties other than the optical ones: forinstance, the silica glass thermomechanical properties, mainly thespecific thermic dilatation may be reduced beyond one magnitude order ifthe glass formulation comprises a titanium oxide fraction. Moreover itis possible to give the silica glass novel functional properties suchas, for instance, the photoluminescence with peculiar emittance andexcitation spectrum selectivities that can be obtained by the presenceof traces of oxides particularly active in the field such as theappropriate excited rare earth oxides.

In the case of use of the manufactured article obtained via theinventive process as a mould, the same submitted to surface treatmentsby means of appropriate antiadhesive agents that permit the removal ofthe product subsequently obtained therein, either optical component ormould again to prosecute the miniaturization fall process.

An example for the preparation of a silica glass mould is given by thesurface silanization to passivate all surface active sites (hydroxylgroups or hydroxyl groups precursors) in order to avoid the adhesion ofthe silicic gel that is aimed to be molded.

EXAMPLES Example No. 1

Diamond Cutting Structure

A. Manufacture of the Original Mould

An original mould was prepared as specified hereinafter. Drawingsaccording to the FIGS. 1 and 2 were supplied by a shop qualified forvery high precision works through the commercially known “turningdiamond” machine tool, by which it is possible to finish off a metalsurface having an average roughness lower than 20 nm. The material tobuild the original mould was an aluminum alloy, commercially known as“CERTAL”.

The structure of the original mould was a 48 mm diameter ring fullycovered by square pyramids having a 2 mm side and about 1.75 mm height.The structured ring was in the center of a metal disk made by aluminumalloy “CERTAL” and having a 56 mm diameter.

A view of the structure is in the FIG. 1 showing, at sizes doubled withrespect to the true ones, the structure layout. The single squarepyramid is illustrated, with strongly enlarged sizes, (10:1) by thelateral view of the FIG. 2.

The structure to be build, integrally formed by rectilinear segments ofdimensions not lower than about 2 mm, was, as far as that kind ofdrawing was concerned, in a size range well compatible with the type ofmachine used. The prototype of the original mould satisfying requiredspecifications, was easily obtained by the encharged shop.

B. Reversed Symmetry Replication Through Siliconic Rubber Imprint

An imprint of the original mould was obtained by using the suitablecompound ELASTOSIL M4601 produced by WACKER CHEMIE GmbH according theusual procedure suggested by the very producer.

C. Miniaturized Replication n.1, in Silica Glass

The siliconic rubber imprint obtained in B was used as a mould for aconventional high precision moulding silicic sol, according to U.S. Pat.No. 5,948,535.

Accordingly, the following operation was carried out: 100 gr of TEOS(tetraethylorthosilicate) were put into a 1000 ml borosilicate glassflask, under a suitable stirring of a magnetic anchor. 300 g of 0.01NHClin bidistilled water were added to TEOS in the flask. The TEOS completehydrolysis was achieved by an ultrasound probe treatment over 10minutes. Hydrolysis ethanol was partially extracted under reducedpressure by a 150 ml volume liquid. 60 g of AEROSIL SiO2, OX-50 DEGUSSAA.G., were added to the residue recovered from the rotating evaporatorand properly homogenized and centrifuged. The silicic sol, before beingpoured into the siliconic rubber mould, was brought to pH 4 through agradual addition of a 0.5N watery ammonium hydroxide.

The sol gelation occurred in about 60 minutes. The obtained gel wasconventionally treated, according to U.S. Pat. No. 5,948,535, convertedto aerogel and densified to silica glass.

The obtained product, as very pure silica glass, was a miniaturizedreplication of the original mould. It is possible to note, through aneye inspection, that the structure of the original mould was faithfullyreproduced in the glass replication with an isotropic linear reductionof all sizes by a factor of about 2.

D. Reversed Symmetry Imprint No. 2

By using the silica glass product obtained in the preceding operation, anovel imprint was prepared according to the procedure previouslydisclosed in B with reference to the original mould.

E. Silica Glass Miniaturized Replication No. 2

The siliconic rubber imprint obtained in D was employed as a mouldaccording to a procedure similar to the one previously specified in theoperation C. The obtained silica glass product was a very good qualityreplication of the original mould structure once miniaturized in C, and,the second time, in E. It is possible to note, through an eyeinspection, that the structure of the original mould was apparentlyfaithfully reproduced with an isotropic linear shrinkage of a factorequal to 4.

The same procedure was applied to reach, through subsequent operationsof siliconic imprint as well as of miniaturization in silica glass, athird and a fourth reduction level of reduction of the original mouldstructure.

The results of the dimensional analyses of all silica glass structuresobtained thereby are summarized in the table 1. TABLE 1 reductionreduction reduction reduction original 1 2 3 4 disk 56.0 28.0 14.0 7.03.5 external diameter (mm) square 2.0 1.0 0.5 0.25 0.125 pyramid side(mm)

The table 1 data outline the inventive level and the industrial utilityof the disclosed invention: a microstructure was obtained through anovel “cascade” process allowing to use the conventional mechanicalprocessing high precision to build the original mould as well as totransfer the same precision to a micrometric scale through the newcascade miniaturized process.

Example No. 2

Microlens Matrix

A. Original Mould Manufacture

An original mould was prepared according to the disclosure of examplen.1, paragraph A, but one difference: the square pyramids having a 2 mmside and a 1.75 mm height of the example 1 were now substituted bytruncated square pyramids, fully equal to the above described pyramids,but being truncated at a 1.6 mm height.

B. Reversed Symmetry Replication Through Siliconic Rubber.

One original mould imprint was obtained by means of the procedurealready disclosed in the paragraph B of the example 1.

C. Silica Glass Miniaturized Replication n.1

An original mould replication was prepared through the procedure alreadydescribed in the paragraph C of the example 1. The silica glass originalreplication, having good optical properties, was similar to the originalone, also having a morphology ideally corresponding to the morphology ofthe original mould, according to one to one correspondence. Only thedimensions were smaller being linearly reduced by a factor of 2.Particularly the base side of the truncated pyramid was reduced to 1 mmand the height to 0.82 mm.

The obtained manufactured article was used in the optical field asmicrolens matrix. Accordingly, an object was put under the groundcontaining the truncated pyramid square bases, 15 mm for therefrom. Theimagine was collected and focused by a lens having the optical axisperpendicular to the base ground on the structure side, sited to see thestructure even if in the opposite side with respect to the object:therefore, vertically above the truncated pyramids.

The results are photographically illustrated in the FIG. 3, showing theobject, constituted by a triangular mark with the number 02, faithfullyreproduced through multiple imagines, each one produced from a differentlens of the circular matrix, i.e. from a truncated pyramid of the moldedstructure.

As a control, the FIG. 4 shows, under the same optical conditions, thesame pyramidal structure, the object having been removed: clearly, thereis no multiple imagine of the object.

The manufactured article, prepared according to this example was used,in the same example, as optical device so called “fly eye”, i.e. as themultichannel lens of an optical system able to show an object at 360degree latitude above a determined horizon. The comparison between theFIGS. 3 and 4 outlines that the result was obtained.

1. An optical article of final or nearly final dimensions, comprisingsilicon oxide, optionally modified by the addition of one or more oxidesof elements other than silicon, having an almost complete isotropy anddimensions equal to or lower than 500 μm.
 2. Process for the preparationof a mould suitable to manufacture an optical article, according toclaim 1, comprising one or more of the following operations that, alltogether, can be continuously carried out as a cascade and/or stopped atthe desired or suitable step: a) preparation of an original highprecision mould; b) reproducing, in a siliconic rubber or other suitablecompound, one or more imprints, having the same sizes and a reversedsymmetry with respect to said mould; c) preparation, by the employmentof one or more of the products obtained in the preceding steps, of theoptical article having reduced dimensions and reversed symmetry withrespect to the starting mould/imprint; according to a sol-gel procedure;d) preparation, inside the so obtained optical article, of a furtherarticle again having reduced sizes and reversed symmetry, or of imprintsaccording to the preceding item b); e) repeating the preparation ofoptical articles by sol-gel procedures and/or imprints according to b),until the desired dimensions or, the lowest dimensions on the ground ofthe physical limitation of the process is achieved; f) separation, inrelation with any step, of the imprint and/or the article obtained inthe very step.
 3. Process for the preparation of a mould according toclaim 2 in which the mould of item a) is produced from nickel/phosphorusalloys on aluminum carriers and aluminum alloys.
 4. Process for thepreparation of the optical article according to claim 2 in which thesol-gel procedure comprises a preliminary step wherein the mould is coolfilled with a sol containing the oxide precursors, the sol is gelled,the gel is dried, the of gel is removed from the mould and the dried gelis miniaturized.
 5. Process for the preparation of an optical articleaccording to claim 4 in which the mould is previously submitted tosurface treatments by means of appropriate antiadhesive agents. 6.Process for the preparation of an optical article according to claim 4in which the mould is previously filled by a silicon oxide precursor. 7.Process for the preparation of an optical article according to claim 6in which the mould is filled also by a precursor of at least an oxide oftitanium, germanium, lanthanides and rare earths.